1. Field of the Invention
The present invention generally relates to a film bulk acoustic resonator and a method of producing the film bulk acoustic resonator, and more particularly, to a film bulk acoustic resonator that has a cavity formed by performing etching with a fluorine-based gas and a method of producing the film bulk acoustic resonator.
2. Description of the Related Art
As mobile communication devices such as portable telephone devices have rapidly become common in recent years, there is an increasing demand for small-sized, lightweight filters formed with devices such as surface acoustic wave (SAW) devices. Especially, having sharp cut-off characteristics and being small and light, surface acoustic wave filters (hereinafter referred to as SAW filters) are being widely used as the RF (Radio Frequency) filters and the IF (Intermediate Frequency) filters of portable telephone devices.
A SAW filter includes a piezoelectric substrate and interdigital transducers (IDTs) formed on a principal surface of the piezoelectric substrate. When an AC voltage is applied to the IDTs, acoustic wave of a certain frequency band is excited on the surface of the piezoelectric substrate.
When a high voltage is applied to the IDTs in such a SAW filter, the IDTs physically break due to deformation of the substrate caused by the acoustic wave. This problem becomes more conspicuous with IDTs having a narrower electrode finger width, i.e., with a higher-frequency filter.
In view of this, SAW filters have the problem of low power resistance, and are difficult to be used as the front-end filters of an antenna duplexer.
To counter this problem, film bulk acoustic resonators (hereinafter referred to as FBARs) have been developed as useful means for forming filters that are resistant to a large amount of power. A FBAR includes a substrate, a piezoelectric thin film, lower and upper electrodes that are metal thin films sandwiching the piezoelectric thin film, and a cavity that is formed under the lower electrode in contact with the substrate. When a potential difference is caused between the upper electrode and the lower electrode in this structure, the piezoelectric thin film sandwiched between the upper electrode and the lower electrode vibrates in the thickness direction by virtue of a piezoelectric effect, thereby showing electric resonance characteristics.
FBARs with such characteristics can be connected in a ladder-like fashion to form a bandpass filter. At present, FBAR filters represented by such a bandpass filter are known to have much greater power durability than SAW filters.
A bandpass filter is required to have low loss in the pass band and to exhibit a high degree of suppression in the stop band. In view of this, the electrode material to be used for the upper electrode and the lower electrode becomes an important factor in producing a bandpass filter with FBARs.
It is essential for an electrode material to have low resistance and high acoustic impedance. The following electrode structures have been suggested to date.
U.S. Pat. No. 5,587,620 (Patent Document 1) discloses an electrode structure that uses molybdenum (Mo) as the electrode material. A film made of molybdenum (Mo) has low resistance and high acoustic impedance. Accordingly, excellent resonance characteristics can be obtained by using a molybdenum film in a film bulk acoustic resonator.
However, molybdenum is easily etched with a fluorine-based gas or an acid-based chemical, and therefore, narrows the variation of the methods of forming a cavity under the lower electrode.
Japanese Patent Application Publication No. 6-204776 (Patent Document 2) discloses one of the methods of forming a cavity under the lower electrode. By this method, anisotropic etching with KOH solution or the like is performed on a single-crystal silicon substrate, so as to form a through hole from the bottom surface of the substrate. Molybdenum. (Mo) cannot be etched with a KOH solution. Accordingly, the lower electrode containing molybdenum (Mo) as the electrode material is not damaged, and a cavity can be easily formed.
Japanese Patent Application Publication No. 2000-69594 discloses another method of forming a cavity under the lower electrode. By this method, a sacrifice layer is deposited in a concavity formed on a substrate. After a lower electrode, a piezoelectric thin film, and an upper electrode are formed, the sacrifice layer is removed. More specifically, a concavity is formed by etching the surface of a silicon (Si) substrate in the first step. In the second step, a thermal oxide film is formed on the surface of the silicon substrate to prevent phosphorus contained in PSG (phosphosilicate glass) to be used as a sacrifice layer from diffusing into the silicon substrate. After the formation of the thermal oxide film, PSG is deposited to form the sacrifice layer in the third step. In the fourth step, mirror finishing is performed on the surface of the deposited sacrifice layer through polishing and cleaning, thereby removing the part of the sacrifice layer outside the concavity. On the surface of the silicon substrate including the sacrifice layer that is exposed through the same plane as the surface of the silicon substrate, a lower electrode, a piezoelectric thin film, and an upper electrode are deposited in this order in the fifth step. In the sixth step, which is the last step, the sacrifice layer is removed to form a cavity under the lower electrode. The removal of the sacrifice layer (the PSG layer) in the sixth step can be carried out with a diluted H2O:HF solution, for example. In this manner, PSG can be etched at a very high etching rate. Accordingly, the cavity can be formed under the lower electrode even though molybdenum (Mo) is used as the electrode material.
The substrate having a through hole formed by the method disclosed in Patent Document 2, however, is very poor in mechanical strength. Because of this, the production yield is low, and it is very difficult to perform the wafer dicing procedure and the package mounting procedure. Furthermore, the through hole formed by anisotropic etching has a tilt angle of approximately 55 degrees. Because of this, it is difficult to connect resonators close to one another in a ladder-like fashion. As a result, a small-sized device cannot be realized.
The method disclosed in Patent Document 3 involves a large number of production procedures, and, by this method, it is difficult to produce a device at a low production cost. Furthermore, the polishing procedure causes problems such as slurry remnants. As a result, the production yield becomes lower.